Radiation-induced free radical polymerization of ethylenically unsaturated monomers is known with both batch and continuous polymerizations having been achieved by this technique. While a desire to build high molecular weight product tends to make batch polymerization methods relatively slow, it generally is desirable to achieve final product in a much shorter time when polymerizing coatings applied to a substrate. Radiation-induced free radical polymerization of coatings currently is accomplished by exposing a polymerizable coating to the output of a high intensity source of radiation, typically a medium pressure mercury lamp or a doped mercury lamp. Photoinitiator(s) present in the coatings absorb certain regions of the spectral output of the lamp generating free radicals and initiating the polymerization process. There is, however, a need to perform radiation-induced free radical polymerization of coatings with greater energy efficiency and with short residence times, particularly on substrates that are adversely affected by heat generated from the radiation sources. Similar considerations are associated with crosslinking of polymeric coatings containing radiation activatable crosslinking agents.
The radiation-induced free radical polymerization of coatings comprising ethylenically unsaturated monomers or oligomers is accomplished by exposing a coating to the spectral output from a high intensity radiation source comprising one or more bulbs that typically have an input power per bulb of about 40 W/cm or greater. These same sources commonly are used for crosslinking polymeric coatings containing radiation activatable crosslinking agents. When a source contains mercury at these power levels, its spectral output occurs over a large range of wavelengths, including the ultraviolet, the visible, and the infrared region of the electromagnetic spectrum. The presence of a radiation activatable species (a photoinitiator for a polymerizable coating or a photocrosslinker for a polymeric coating) generally is required for these systems to work. Photoinitiators and photocrosslinking agents do not, in general, absorb appreciably in either the visible or the infrared regions of the spectrum and therefore only a small percentage of the total spectral output from these sources is used to induce polymerization of a polymerizable coating or to crosslink a polymeric coating. Further, differential absorption by the radiation activatable species within a coating causes large cure gradients to form between its top and bottom surfaces. Infrared radiation and radiant heat often also are undesirable themselves as their presence causes low molecular weight species to volatilize from coatings, distorts heat sensitive backings, and requires shutters and/or safety interlocks to minimize the potential for fire.
Processes that employ energy more efficiently recently have been introduced. Stark and Wright in WO 97/39837 describe use of a monochromatic light source, preferably a xenon chloride excimer lamp having a peak wavelength of 308 nm, to crosslink a hot melt coatable adhesive composition. In these compositions, the absorptivity of the photoactivatable crosslinking agent at the wavelength of the excimer source is low allowing for the penetration of the light through the entire coating thickness causing a minimal cure gradient to form.
Wright in WO 97/40090 describes use of a monochromatic light source, preferably a krypton chloride excimer lamp having a peak wavelength of 222 nm, to cure free radically polymerizable coatings without the necessity of a photoinitiator. When photoinitiators are present, the light is not used efficiently due to competing absorption by the polymerizable monomers and oligomers.
Nohr and MacDonald in WO 96/00740 describe a general method for generating a reactive species by providing a polymolecular photoreactor comprising a wavelength specific sensitizer in association with a reactive species generating photoinitiator and irradiating the sensitizer with actinic radiation. In one embodiment, an excimer lamp is employed as the radiation source.
There remains a need to perform radiation-induced polymerization of free radically polymerizable coatings containing a photoinitiator and a need to perform radiation induced crosslinking of polymeric coatings containing a photoactivatable crosslinking agent with greater energy efficiency and with shorter exposure times.